1. Technical Field of the Invention
The present invention relates generally to a dc motor equipped with permanent magnets used as field magnets, and more particularly to an improved structure of such a motor designed to have enhanced startability thereof.
2. Background Art
Japanese Patent Second Publication No. 5-10903 teaches techniques for improving the speed of a dc motor equipped with permanent magnets in a low-current range without sacrificing the torque output in a high-current range in order to enhance the startability thereof at room temperature. Specifically, the structure, as taught in such a publication, has magnetic material-made auxiliary poles each of which is disposed on a magnetizing side of one of permanent magnets working as main poles where the armature reaction is developed, thereby increasing effective magnetic fluxes which are produced from the auxiliary poles and link with armature coils to increase the output of the motor.
The above structure, however, has the drawback in that the auxiliary poles are magnetized during energization of the armature coils to produce magnetic attraction between the auxiliary poles and an armature, thus requiring the need for securing the auxiliary poles to a yoke against the magnetic attraction. For example, it is necessary to weld the auxiliary poles to the yoke or to place a sleeve on an inner circumferential side of each of the auxiliary poles.
Additionally, as compared with motors equipped with the auxiliary poles, the above dc motor also has the problems of increases in number of component ports and production cost thereof.